1. Field of the Invention
The present invention relates generally to envelope generators and keyer circuits used in electronic organs. More specifically, the preferred embodiment of the present invention relates to an envelope circuit which can be adjusted to compensate for process variations occurring during the manufacture of an integrated envelope generator and keyer circuit.
2. Description of the Prior Art
Conventional circuits for electronic organs have tone generator circuits for providing a signal having a selected frequency, typically 60-12,000 Hz, and envelope generator circuits for providing a signal characterized by a maximum amplitude, a rise or attack time to reach that amplitude once a key has been depressed, and a fall or decay time for the signal to decay from the maximum amplitude to a zero amplitude once the key has been released. These two signals are mixed in response to the activation of an organ key to produce the desired audio output frequency signal. However, process variations in the manufacture of integrated circuits affect the value of the maximum amplitude, the attack time and the decay time in different ways. It is, therefore, desirable to compensate integrated organ circuits so that the amplitude, attack and decay characteristics stay within desired tolerances.